1. Field of the Invention
The present invention relates to a composition comprising a radical polymerization inhibitor whose kind and content vary to simultaneously form two column spacers having different shapes through a slit or semi-transmissive mask. The present invention also relates to column spacers formed using composition and a liquid crystal display comprising the column spacers.
2. Description of the Related Art
A liquid crystal cell of a liquid crystal display essentially includes a thin film transistor (TFT) substrate for driving the liquid crystal display, a color filter substrate for color display, and a liquid crystal filled between the two substrates. The color filter substrate is produced by subjecting a photosensitive organic material, in which at least one pigment is dispersed, to photolithography to form a pattern and coating three or more kinds of color inks having different transmission-absorption wavelengths on the pattern to form pixels. The pixels are combined to create color images. If needed, overcoats are formed in the color filter substrate to reduce the step between the adjacent pixels, or column spacers are formed by patterning to maintain the inner gap of the liquid crystal cell at a constant level.
Photosensitive resin compositions, particularly, negative type photosensitive resin compositions patternable by photolithography are usually used to form the overcoats or the column spacers. A typical negative type photosensitive resin composition essentially comprises an alkali-soluble polymer, a polyfunctional monomer having two or more acrylate groups, and a photoinitiator. Optionally, the negative type photosensitive resin composition may further comprise a solvent, a surfactant, an adhesion aid, etc.
The photoinitiator is decomposed to generate active radicals when the negative type photosensitive resin composition is exposed to light, particularly UV light. The active radicals activate the acrylate groups of the polyfunctional monomer to induce cross-linking of the alkali-soluble polymer with the polyfunctional monomer (photopolymerization). This photopolymerization increases the molecular weight of the polymer in the cross-linked portion (i.e. in the portion exposed to light), and as a result, the exposed portion becomes insoluble in alkali. The exposed portion remains unremoved even after subsequent development. Consequently, the use of the negative type photosensitive resin enables the formation of a fine pattern by photolithography.
The sensitivity of a photosensitive resin composition is associated with the minimum exposure dose (i.e. light energy) at which a pattern is stably formed. A low exposure dose shortens the processing time to assist in productivity improvement. Particularly, the sensitivity of a photosensitive resin composition for forming column spacers to maintain the gap of a liquid crystal cell at a constant level is determined as an exposure dose at which the variation in the thickness of patterns is sharply decreased.
Photoinitiators that rapidly respond to low light energy have been used to improve the sensitivity of photosensitive resin compositions without any marked reduction in the developability of the photosensitive resin compositions. However, the use of photoinitiators for improving the sensitivity of photosensitive resin compositions is disadvantageous in that the variation in the thickness of patterns depending on the exposure dose is small. This disadvantage makes it difficult to form transparent thin films whose thickness is controlled by varying the slit structure and transmittance of photomasks.
Radical polymerization inhibitors are often added to negative type photosensitive resin compositions. The radical polymerization inhibitors may be added separately. In most case, the radical polymerization inhibitors are present in small amounts in other raw materials, such as binder resins, cross-linking agents and solvents.
Radical polymerization inhibitors have been used for the purpose of improving the storage stability (stability over time) of photosensitive resin compositions because of their ability to remove a small amount of radicals created during storage of the photosensitive resin compositions.
A conventional negative type photosensitive resin composition enables the formation of column spacer patterns having the same shape, but it has difficulty in simultaneously forming a saturated pattern and a semi-transmissive pattern as column spacer patterns having different shapes for use in a liquid crystal display through a slit or semi-transmissive mask.
Thus, there is an urgent need to develop a composition for simultaneously forming a saturated pattern and a semi-transmissive pattern as column spacer patterns having different shapes, whose difference in thickness is controllable as desired although the sensitivity is slightly reduced, for use in a liquid crystal display through a slit or semi-transmissive mask.